1. Field of the Invention
The present invention relates to a random number generating circuit for generating a physical random number, and, for example, to a random number generating circuit suitable to generate a random number used to produce a code key required for security functions of an IC card, a stored-program one-chip microcomputer, and the like.
2. Description of the Related Art
A random number generating circuit used to secure an ID, a code key, or the like requires high randomness.
A random number is broadly divided into a physical random number generated based on a random phenomenon which occurs in nature and a pseudo random number which is artificially generated.
The pseudo random number is a random number artificially generated by a logic circuit or software. The pseudo random number is typically generated by, for example, a built-in random number circuit of a personal computer.
However, in the case of the pseudo random number, a random number generation procedure is determined by the logic circuit or the software. Therefore, when an initial system state is found, the random number can be relatively easily predicted, so there is a case where the security is incomplete.
On the other hand, the physical random number is normally assumed to be a cryptographically safe random number because of high randomness.
Examples of the physical random number include an electrical thermal noise of a resistor and a shot noise of a PN junction of a semiconductor.
A technique of sampling a thermal noise generated by a thermal noise element, storing the sampled thermal noise as a charge in a capacitor unit, amplifying a voltage corresponding to the charge stored in the capacitor unit, and converting the amplified voltage into a digital signal by A/D conversion to generate a random number is employed for the random number generating circuit using the physical random number (see, for example, JP 2001-175458 A).
The above-mentioned physical random number is generated based on a thermal noise or the like from a noise source. However, a noise level is slight (several tens μV to several hundreds μV), so a high voltage is required to extract the thermal noise as an effective random number.
With an increase in density of circuits due to improvement of LSI functions, a reduction in manufacturing element size is being advanced. When the element size reduces, there is a problem in that an element withstand voltage reduces, thereby reducing a circuit withstand voltage.
In view of the security, it is necessary to incorporate the random number generating circuit in an LSI chip. However, the structure using the physical random number is used, so there are a large number of problems to be solved, which include a problem related to suitability as an LSI material.
In particular, the conventional random number generating circuit includes a large number of elements for realizing a specific structure of a noise amplifier serving as a unit for extracting a random noise component. Therefore, there arise problems in that an area of the LSI chip increases and a manufacturing cost increases.